FIX: improve output path and store metadata [#873]

[gjheij]

Fixes # .

Changes proposed in this pull request:

• Directly check if specified argument save_path is a string; if not, set to default
• Add all fields from behavioral json to ses.infos, to ensure required fields for pupil analysis are populated

        fn = fieldnames(beh_json);
        for ii = 1:numel(fn)
            ses.infos.(fn{ii}) = beh_json.(fn{ii});
        end